1. Field of the Invention
The invention relates to a method for controlling a hydraulic vehicle drive, in which a variable displacement pump, which is driven by a diesel engine, conveys hydraulic fluid in a closed circuit to and from a hydraulic motor, which drives the driving wheels of the vehicle.
The invention also relates to an arrangement for controlling a hydraulic vehicle drive, having a closed hydraulic circuit which has a variable displacement pump, which is driven by a diesel engine. A hydraulic motor drives the driving wheels of the vehicle, and two lines which connect the variable displacement pump and the hydraulic motor are the feed or return line for the hydraulic motor, depending on the direction of travel of the vehicle.
2. Description of the Related Art
If the variable displacement pump and the hydraulic motor of prior art systems are set up for two conveying directions, the vehicle can be changed over from forward to rearward travel by the direction of flow of the hydraulic fluid being reversed. Furthermore, the hydraulic vehicle drive can also act as a brake, if the vehicle changes into braking or overrunning operation. In this case, the diesel engine which drives the variable displacement pump changes to idling, and the conveying flow of the variable displacement pump is reduced to zero by means of a reduction in the pump pivot angle. However, the traveling speed remains unchanged initially as a consequence of the inertia of the moving vehicle. The power flow from the variable displacement pump to the hydraulic motor or motors then turns around, and the hydraulic motor operates as a pump. The variable displacement pump then acts as a hydraulic motor. The direction of flow of the hydraulic fluid remains unchanged, but it is now conveyed by the hydraulic motor in the direction of the variable displacement pump. The variable displacement pump builds up hydraulic resistance, and the operating pressure (braking pressure) of the closed hydraulic circuit is increased as a result. This produces a torque which attempts to accelerate the diesel engine in the braking or overrunning operation. If the nominal rotational speed of the diesel engine is substantially exceeded, the latter may be damaged, in particular if it is a diesel engine which is provided with a turbocharger.
In some prior art systems, a pressure regulating valve acts as a restrictor when the hydraulic fluid flowing back from the hydraulic motor is at an increased pressure. As a result, a targeted conversion of energy from flow energy into heat takes place, and the braking energy is correspondingly reduced. Diesel engine damage stemming from the braking or overrunning operation is then ruled out. However, this system has the disadvantage that the conversion of flow energy into heat takes place regardless of which pivot angle is set at the variable displacement pump. Therefore, this takes place regardless of the torque which the variable displacement pump can deliver to the diesel engine. As the pivot angle of the pump decreases, the torque is reduced at constant pressure, so that the braking potential of the diesel engine cannot be used optimally. It is also disadvantageous that the entire flow of hydraulic fluid is conducted via the pressure regulating valve, which acts as a restrictor. Therefore, during the restricting of the conveying flow, the entire restricted flow energy is converted completely into heat. This results in an undesirable increase in the temperature of the hydraulic fluid.
Therefore, a principal object of this invention is to provide a system for controlling a hydraulic vehicle drive in such a manner that as little flow energy of the hydraulic fluid as possible is converted into heat, and that the torque of the diesel engine is used optimally in the overrunning operation.
The design according to the invention is based on the consideration that the vehicle drive is configured for maximum conveying flow even for the braking and overrunning operation. Since, however, as the traveling speed decreases the amount of conveyed hydraulic fluid is reduced, the restricting effect of the closed circuit becomes lower, and so a higher pressure can build up in the line upstream of the variable displacement pump. As a result, as the traveling speed decreases, the difference in pressure at the variable displacement pump increases. That portion of flow energy which has still to be converted into heat is reduced thereby, since the reduced difference in pressure becomes smaller overall.
The novel method of this invention comprises a sequence valve responding to the pressure, which is produced by the hydraulic motor, to control the pivot angle of the variable displacement pump. At least one pressure regulating valve and a main circular orifice are arranged in parallel in the line downstream of the sequence valve, in the direction of flow of the hydraulic fluid. This arrangement provides that the product of the displacement volume of the variable displacement pump and of the pressure of the hydraulic fluid in the line upstream of the variable displacement pump remain approximately constant.
A further advantageous refinement of this consists in a one-way or check valve being arranged in parallel connection to the constructional unit consisting of the sequence valve, a restrictor or an orifice, at least one pressure regulating valve and a main circular orifice. The check valve blocks the throughflow of the hydraulic fluid in the direction from the hydraulic motor to the variable displacement pump, but releases or allows it in the opposite direction. This refinement achieves the result that in spite of the built-in parts in the high pressure line for the braking operation, undisturbed operation is possible at any time in the reverse direction of travel. In this case, the hydraulic fluid bypasses the sequence valve, orifice and pressure regulating valve and flows through the opened check valve from the variable displacement pump to the hydraulic motor. The main circular orifice brings about a targeted restriction of the hydraulic fluid in such a manner that, in the braking or overrunning operation and at maximum vehicle speed, the pressure regulating valve immediately closes.
A further advantage of the instant invention is that the arrangement is performed in such a manner that in the braking operation the variable displacement pump is directed by the sequence valve to a relatively large pivot angle if the sequence valve, by reaching the set increased pressure, is responding for the first time. After that, the pivot angle is then restored stepwise because the pressure in the line upstream of the variable displacement pump increases because the conveying amount is becoming smaller.
This can be configured purely hydraulically. However, a combination of electronics and hydraulic circuitry is also possible in that the rotational speed of the diesel engine being incorporated as the influencing variable. This is accomplished by an electrically proportional sequence valve which is controlled by an electronic control device as a function of rotational speed of the diesel engine, which controls the pivot angle of the variable displacement pump via a pump bypass line, if the increased pressure is present in the pump bypass line.